The present invention relates to a photodetector used for a hologram-type optical head of an optical disk apparatus.
FIG. 3 shows an example of constitution of a hologram-type optical head which performs a double-knife-edge-type focusing control by utilizing a hologram device. As shown in FIG. 4, a conventional photodetector used for this kind of hologram-type optical head comprises 6-piece-divided photodiodes which include 4-piece-divided photodiodes A, B, C and D for focusing control and 2-piece-divided photodiodes E and F for tracking control which are disposed outside the 4-piece-divided photodiodes. As shown in this figure, a focusing error is detected by the 4-piece-divided photodiodes, that is, mere four long and narrow photodiode elements A, B, C and D.
In the hologram-type optical head shown in FIG. 3, a laser light beam emitted from a laser diode 1 is split by a diffraction grating 2 into three light beams, and the split three light beams pass through a hologram 3, are focused by an object lens 4, and form spots on a signal surface of an optical disk 5. Return light beams reflected from the disk 5 return to the hologram 3 along the same paths as the incident paths. The hologram 3 comprises divided two areas which have respective grating patterns different from each other so that each diffracted light beam is focused on two different points on a 6-piece-divided photodetector 6 as shown in FIG. 4. In this case, a focusing error signal (F.E.), a tracking error signal (T.E.), and a reproduced RF signal (R.F.) are expressed as following respective equations: EQU F.E.=(A+C)-(B+D) EQU T.E.=E-F EQU R.F.=A+B+C+D
where A to F represent outputs of the corresponding photodiodes.
By the way, the wavelength of laser light beam emitted from the laser diode varies with temperature. If the wavelength of laser light beam varies, a diffraction angle by the hologram 3 is accordingly varied, whereby the spots of diffracted return light beams on the 6-piece-divided photodetector 6 are shifted in a x-direction in FIG. 4. In order to prevent the photodetector 6 from receiving the influence of the wavelength variation of the laser diode, the photodiode elements A, B, C, D, E and F have long and narrow shapes. In this case, the two spots of each return light beam on the photodetector 6 should be adjusted so as to locate on their design positions (normally, the centers of the 4-piece-divided photodiodes, that is, the positions on lines (a) shown in FIG. 4). However, even if the spot positions of the return light beam are deviated, an output of each photodiode element does not vary. Therefore, it could not be performed to adjust the spot positions of the return light beams to the centers of the photodiode elements.